J. Cent. South Univ. (2019) 26: 3436-3456

DOI: https://doi.org/10.1007/s11771-019-4265-6

Depositional conditions and modeling of Triassic Oil shale in southern Ordos Basin using geochemical records

LI De-lu(ÀîµÃ·)^{1, 2, 3}, LI Rong-xi(ÀîÈÙÎ÷)⁴, TAN Cheng-qian(Ì·³ÉǪ)⁵, ZHAO Di(ÕÔµÏ)⁴,LIU Fu-tian(Áõ¸£Ìï)⁴, ZHAO Bang-sheng(ÕÔ°ïʤ)⁴

1. Shaanxi Provincial Key Laboratory of Geological Support for Coal Green Exploitation, Xi¡¯an University of Science and Technology, Xi¡¯an 710054, China;

2. Geological Research Institute for Coal Green Mining, Xi¡¯an University of Science and Technology,Xi¡¯an 710054, China;

3. College of Geology and Environment, Xi¡¯an University of Science and Technology, Xi¡¯an 710054, China; 4. School of Earth Sciences and Resources, Chang¡¯an University, Xi¡¯an 710054, China;

5. School of Earth Sciences and Engineering, Xi¡¯an Shiyou University, Xi¡¯an 710065, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: Based on the element geochemistry and biomarkers of the oil shale from the Chang 7 sub-unit in the southern Ordos Basin, the depositional conditions and organic source of the oil shale are discussed. Biomarkers analyses show that the oil shale has a homologous organic matter source, with a mix of plankton and advanced plants. U/Th and V/Ni ratios suggest that the redox condition is dominated by a reducing condition, and the degree of anoxia in the Tongchuan area is higher than that of the Xunyi area. Sr/Ba ratios illustrate that the oil shale is deposited in fresh water and the paleosalinity in the Tongchuan area is slightly higher. Fe/Ti ratios imply that the Tongchuan area underwent obvious hydrothermal fluid activities. Sr/Cu ratios show warm and humid paleoclimate in both areas. As assessed by (La/Yb)_NASC, the deposition rate in the Tongchuan area is relatively lower. Fe/Co and Th/U ratios suggest that the paleo-water-depth in the Tongchuan area is deeper. The source rock could have the advance plants source, which must have close relationship with the Qinling orogeny. Comparing the paleoenvironment, the Tongchuan area has better depositional conditions, and is the key oil shale exploration area in the southern Ordos Basin.

Key words: oil shale; geochemistry; depositional model; Chang 7; Ordos Basin

Cite this article as: LI De-lu, LI Rong-xi, TAN Cheng-qian, ZHAO Di, LIU Fu-tian, ZHAO Bang-sheng. Depositional conditions and modeling of Triassic Oil shale in southern Ordos Basin using geochemical records [J]. Journal of Central South University, 2019, 26(12): 3436-3456. DOI: https://doi.org/10.1007/s11771-019-4265-6.

1 Introduction

Oil shale is a type of fine grained sediment with high organic matter; it can release shale oil after combustion [1-3]. Due to the dwindling of conventional oil resources, oil shale, as an unconventional alternative resource, has received unprecedented attention [4-8]. The most studied oil shale in the world is the lacustrine oil shale of the Green River Basin in the United States, which has the largest amount of resources [1, 9, 10]. Chinese oil shale is primarily distributed in the Songliao Basin and Ordos Basin; the oil shale from the Triassic Chang 7 sub-unit in the southern Ordos Basin is characterized by shallow burial depth, extensive distribution, and relatively high oil yield [1, 11-13].

Studies indicate that the organic matter of oil shale from the Chang 7 sub-unit is dominated by Type I and II, and the origin of the organic matter is primarily algae, with low maturity [14, 15]. After analysis of a large data set obtained from boreholes and a geology survey, LUO et al [16] suggested that the thickness of the oil shale varies from 10 to 40 m. The oil shale is deposited in fresh water under 150 m maximum depth [17]. SUN et al [18] considered that lake flooding, together with anoxic events, volcanic movements, and turbidity events have important effects on enrichment, preservation, abundance, and the type of organic matter in oil shale. After interpretation of a large number of well logs, DENG et al [19] forecasted that the Tongchuan and Xunyi areas in the southern Ordos Basin have favorable potential for oil shale development. The sedimentary environment affects the elemental geochemical behavior of sedimentary rocks, and vice versa; elemental geochemical characteristics can also reflect the ancient environment of the sedimentary period [20-22]. Elemental geochemistry is an effective means of paleo-environment reconstruction. The redox condition, salinity, hydrothermal fluid activities, climate, sedimentary rate, and water depth can be recovered by the element ratios and their related calculation, semi-quantitatively or quantitatively [3, 23-25]. These values have a close relationship with enrichment and preservation of organic matter [26, 27]. Compared to qualitative analysis of the paleo-environment by rock combinations and organic geochemistry, elemental geochemical recovery is more accurate.

In addition to being used for oil and source rock correlation, biomarkers have implications for the source of organic matter in source rocks [3, 28-30]. YANDOKA et al [31] suggested that regular sterane (C₂₇, C₂₈ and C₂₉) can indicate different sources of organic matter, and found that the organic matter of Cretaceous sediments in the Yola Sub-basin was dominated by planktonic/ bacterial matter. HAKIMI et al [4] considered the relative amount regular sterane, and found that the organic matter of Mesozoic oil shale in central Jordan was primarily composed of plankton/algal matter and microorganisms. Thus, understanding of the source of organic matter can be achieved through analysis of the regular sterane in biomarkers.

Up to now, in most lacustrine basins, the organic matter source of the superb source rock often originates from inferior algal organism. The oil shale from Green River Formation in American has been verified that the lower biological debris contribute to the organic matter in the oil shale [32, 33]. The organic matter type of the fresh water oil shale in Huadian in northeastern China is dominated by Type I, reflecting that organic matter source is mainly inferior algal organism [34]. The Permian oil shale of Junggar Basin in western China was formed in a closed freshwater lake with the lower aquatic organisms as the main organic matter source [35, 36]. It is worth noting that the above lacustrine oil shales are almost deposited in a relatively stable sedimentary environment. The researches on the oil shale in complex tectonic environment are still insufficient.

Oil shale outcrops and boreholes from the Chang 7 sub-unit in the Tongchuan and Xunyi areas have similar physical properties with a huge amount of resources, over 14159.04¡Á10⁸ t [13]. According to sedimentary facies, the Tongchuan and Xunyi areas are all located in semi-deep to deep lake, facilitating oil shale formation [37]. However, the average thickness and oil yield in Tongchuan area are around 30 m and 5%-10% [22]. While, the thickness and oil yield of the oil shale in Xunyi area are obviously smaller than that in Tongchuan area. This is likely to be associated with the depositional conditions. Moreover, the total organic matter of the oil shale from the Tongchuan area is slightly higher than that from the Xunyi area [13,15], and complex geological events happening in the southern Ordos Basin during oil shale sedimentation are likely to have a different effect on depositional conditions [3, 38, 39]. Therefore, it is necessary to investigate the depositional conditions of the two areas in southern Ordos Basin to find out the reasons for the differences in the thickness, oil yield and total organic matter. This can provide theoretical direction for further oil shale exploration in the southern Ordos Basin.

In this work, oil shale samples from the Tongchuan and Xunyi areas in the southern Ordos Basin are analyzed with regard to major elements, trace elements, and rare earth elements (REEs), and gas chromatography-mass spectromete (GC-MS) is performed. The organic matter source of the oil shale is discussed and the paleo-environment of the oil shale in these two areas is compared. Finally, a depositional model of oil shale from the Chang 7 sub-unit is proposed.

2 Geological setting

Located in midwestern China, the Ordos Basin is a superimposed basin with stable deposition and multiple sedimentary cycles [30, 40, 41]. Before the Carboniferous-Permian period, the basin belonged to the marine basin of the North China Block [39], and then gradually departed from the North China Block and evolved into a large inland sedimentary basin [40, 41]. In the Early Cretaceous, the whole basin uplifted and underwent later reformation. According to the present tectonic characteristics and evolutionary history, the basin is divided into six first-order tectonic units, including the Weibei Uplift, Yishan Slope, Yimeng Uplift, Jinxi Flexure Zone, Tianhuan Depression, and Western Thrusted Zone (Figure 1(a)). The Triassic Yanchang Formation primarily developed delta and lake facies, and went through an integrated occurrence- extinction period, depositing a set of progradation- aggradation-retrogradation stratum, with a thickness of 1000-1300 m [41-43]. According to the sedimentary cycles and rock assemblages, the Yanchang Formation is divided into 10 sub-units [44], reflecting a whole transgressive¨Cregressive lacustrine cycle [39, 41]. In particular, during the initial stage of the Chang 7 period, the lake reached its largest scale, and together with tectonic orogenies and paroxysmal eruption in the south [22, 39] and formed a high quality of oil shale characterized by a huge thickness and high organic content. This was named the Zhangjiatan Oil Shale [45, 46]; it is located at the bottom of the Chang 7 sub-unit, and contributes to the entire Mesozoic oil reservoirs in the Ordos Basin [19, 47, 48].

Figure 1 Geological map of Ordos Basin (modified after LI et al [22]), sedimentary facies of southern Ordos Basin (modified after YANG et al [37]), and location of study area, showing that study area is primarily located in the Weibei Uplift, and the depositional sub-facies is semi-deep to deep lake (a) and stratigraphic column of Upper Triassic Yanchang Formation in study area (modified after LI et al [22]), showing that whole Yanchang Formation represents evolutionary process of the lake, from start to end. Chang 7 period comprises the largest lake with premium oil shale (b)

The study area is primarily located in the Weibei Uplift of the southern Ordos Basin. Due to the Indosinian orogeny and the Yanshan Orogeny, the study area underwent unbalanced uplift, and missed the upper stratums in the Yanchang Formation with the current stratum of Chang 10-Chang 6. Previous studies have indicated that the southern Ordos Basin is the depocenter of the lake and the sub-facies is semi-deep to deep lake during Chang 7 period [22, 37] (Figure 1(a)). The bottom of the Chang 7 sub-unit primarily developed oil shale, shale, mudstone, and silty mudstone (Figure 1(b)).

3 Sampling and analytical methods

A total of 26 oil shale samples were collected in the Tongchuan and Xunyi areas and the oil shale samples were all from the bottom of Chang 7 sub-unit. Twenty of them were selected from 4 boreholes scattered in southern Ordos Basin and sent to analyze major elements, trace elements and rare earth elements (Figure 2). Six of them were analyzed by GC-MS, in which three were collected from Yishicun profile in the Tongchuan area and the other three were collected from three well drillings in the Xunyi area (Figure 1(a)).

The samples for element analysis were all powdered to less than 75 ¦Ìm, using X-ray fluorescence spectrometry (XRF) for major elements and inductively-coupled plasma mass spectrometer (ICP-MS) for trace elements and rare earth elements. For XRF, the ashed samples were first sent to an open muffle furnace with the mounting temperature of 950 ¡ãC and then were dissolved with HClO₄ and HF. The operation was conducted by AA-6800 atomic absorption spectroscopy and UV-2600 ultraviolet-visible spectrophotometer. For ICP-MS, the ashed samples were digested in microwave furnace by an HF+HNO₃ mixture liquid in Teflon bombs at 190 ¡ãC for 48 h. The operation was conducted by Perkin Elmer SciexElan 6000. The analytical procedures followed Chinese National Standard GB/T14506.1-14-2010 [49] and GB/T14506.30- 2010 [50], respectively. The analytical error was within 1%. The analyses were performed at the Analytical Center, No.203 Research Institute of Nuclear Industry.

For gas chromatography-mass spectrometry (GC-MS) analyses, samples were extracted with chloroform (CHCl₃) for 72 h after being powdered under 80 mesh and silica gel, together with alumina column, which was used to separate group composition. Saturated hydrocarbon, aromatic and nonhydrocarbon were obtained with n-Hexane, benzene and ethanol as solvent. Corresponding biomarkers were performed on an Agilent 6890GC/5975i MS with a 60 m¡Á0.25 mm¡Á0.25 ¦Ìm HP-5MS fused silica capillary column under condition of 26 ¡ãC and 50% relative humidity (70 eV ionization voltage, 100 mA filament emission current, 150-250 ¡ãC interface temperature), with He as the carrier gas (30 m¡Á 0.32 mm). The gas was boosted at the rate of 1.0 mL/min and the start temperature of the oven is 50 ¡ãC. Then the oven was hearted to 310 ¡ãC at the rate of 20/min after 1 min, then held at 310 ¡ãC for 25 min. Testing standard was followed by GB/T 18606-2001 [51]. The experiments were conducted at Key Laboratory of Exploration Technologies for Oil and Gas Resources, Ministry of Education, Yangtze University, China.

4 Geochemistry characteristics

4.1 Elements geochemistry

The results of the major elements analysis of the oil shale samples are listed in Table 1. The concentration of SiO₂ (36.42 wt%-64.70 wt%) comprised almost half of the total concentration of major elements, and the concentrations of Al₂O₃ (10.69 wt%-20.15 wt%) and TFe₂O₃ (3.61 wt%- 11.27 wt%) were also relatively high. The content of the rest of the major elements was very low, with averages of less than 5%. The element Si is enriched in oil shale samples, indicating that quartz is likely to be present (Figure 3(a)). This is in accordance with XRD analysis [22]. The elements Al and Ti are relatively stable and can be used to judge the parent rock type because of the low solubility of their oxides and hydroxides [52]. The Al₂O₃/TiO₂ ratio is regarded as very contiguous to that of the parent rocks. Al₂O₃/TiO₂ ratios ranging in 3-8, 8-21 and 21-70 are indicative of mafic igneous rocks, intermediate rocks, and felsic igneous rocks, respectively [53]. The Al₂O₃/TiO₂ ratios of the oil shale samples varied from 21.76 to 46.69, indicating that the parent rock of the oil shale is felsic igneous rock (Figure 3). The Al₂O₃/TiO₂ ratios of the oil shale samples in the Tongchuan area (28.8-46.7, averaging 34.5) were slightly higher than those of the Xunyi area (21.7-33.3, averaging 27.3); this may be due to the paleo-environment. Si/(Si+Al+Fe) is a reflection of distance from the terrigenous provenance, and decreases with increasing distance [54]. The average Si/(Si+Al+Fe) ratios of the oil shale samples in the Tongchuan (0.65) and Xunyi areas (0.62) were relatively small, with little variability, suggesting that the oil shale was formed near the terrigenous provenance.

Figure 2 Photos (a) and sketches (b) of oil shale sections from four boreholes and sampling locations in the Chang 7 sub-unit. The length of sample boxes is 1 m and the diameter of the core columns is 7 cm. The top of the oil shale section is located in the upper left corner and the bottom is in the lower right corner. Oil shale portions are layered with lamina tuff and massive microstructures

Table 1 Concentrations of major elements and associated geochemical parameters of oil shale samples from Chang 7 sub-unit

Figure 3 Rock type ternary diagram, showing relative proportions of major elements SiO₂(quartz), Al₂O₃(clays), and CaO(carbonates) in oil shale samples. This shows that the Si concentration in oil shale is relatively high and quartz is dominant mineral. The base map is from LI et al [26]

The trace element contents of the oil shale samples are presented in Table 2. Compared with the upper continental crust (UCC), Cu and U were enriched in oil shale samples (Figure 4), and enrichment factors (EF, F_e) (F_e=X_samles/X_ucc) of Cu and U of oil shale samples in the Tongchuan area (Cu: 1.98-6.00, average 5.01; U: 7.14-24.36, average 17.87) were higher than in the Xunyi area (Cu: 1.25-3.90, average 2.30; U: 1.05-7.71, average 3.60) (Figure 4 and Table 3). Sr, Ba, V, Th, Co, Ni, and Sc of the oil shale samples in the two areas show little difference. The element Cu is often associated with organisms, and the enrichment of Cu can represent the prosperity of living matter in a lake [56]. In an oxidation state, the element U usually exists in the +6 valence state, and is inclined to dilute, while in a reducing state, it is poorly soluble in water and tends to gather in the form of the +4 valence state. Therefore, the extremely concentrated types of Cu and U are closely associated with high abundance of organic matter and a reducing condition [57, 58]. Therefore, the difference in Cu and U concentration is closely related with paleo-productivity and paleo-redox conditions.

Table 2 Trace elements contents of samples and corresponding UCC values of oil shale samples from Chang 7 sub-unit (¡Á10^-6)

Figure 4 Spider diagram of trace elements of oil shale samples, indicating that oil shale from Tongchuan area is more enriched with U, Cu, and V elements, as compared to Xunyi area, which is likely related to depositional environment

The concentrations of rare earth elements in the oil shale samples are shown in Table 4. The total average content of rare earth elements (¡ÆREE) of oil shale samples in the Tongchuan area (160.1 ¦Ìg/g) is less than that in the Xunyi area (192.4 ¦Ìg/g). The light rare earth elements (¡ÆLREE) and heavy rare earth elements (¡ÆHREE) of the oil shale samples in the Tongchuan area (¡ÆLREE: 123.9-180.7 ¦Ìg/g, averaging 143.6 ¦Ìg/g; ¡ÆHREE: 14.23-25.65 ¦Ìg/g, averaging 16.52 ¦Ìg/g) are mostly lower than those in the Xunyi area (¡ÆLREE: 142.6-207.9 ¦Ìg/g, averaging 175.2 ¦Ìg/g; ¡ÆHREE: 15.36-20.79 ¦Ìg/g, averaging 17.17 ¦Ìg/g). Moreover, the ¡ÆLREE/¡ÆHREE (L/H) of oil shale samples in the Tongchuan area (7.05-10.15, averaging 8.82) is lower than that in the Xunyi area (9.09-11.71, averaging 10.21).

Table 3 Enrichment factors of trace elements of oil shale samples from Chang 7 sub-unit

Eu anomaly (¦Ä_Eu) is the result of the differentiation of elements, and can reflect the parent rock properties [21, 24]. ¦Ä_Eu is regarded as a positive anomaly and negative anomaly when >1.05 and <0.95, respectively. Upper crustal rocks are normally characterized by Eu negative anomaly [61]. After being normalized by chondrite [60], all of the oil shale samples in the Tongchuan and Xunyi areas are well below 0.95, suggesting that the parent rocks of the oil shale come from continental crust. Ce anomalies (¦Ä_Ce) of the oil shale samples were close to 1, indicating no obvious abnormality.

The similar North American shale (NASC)- normalized and chondrite-normalized distribution patterns indicate that oil shale samples in the two areas may have formed in an appropriate environment with similar provenance (Figure 5). However, in consideration of the non-negligible difference in concentration of REEs and L/H ratios of oil shale samples of these two areas, the paleo-environment in these two areas is probably the main influence on the discrepancy during oil shale sedimentation. It is noteworthy that heavy rare earth elements show a slightly rich trend, which may be influenced by the absorption of organic matter [62].

4.2 Biomarkers

In this study, sterane distribution of six oil shale samples was examined by m/z 217 mass chromatograms, and relative amounts of biomarkers were calculated by measuring retention time and peak heights. Peak assignments are listed in Appendix A.

Regular sterane and diasterane are very important for researching maturity and source of organic matter [29, 63-65]. ¦Á¦Á¦ÁC₂₉sterane20S/ (20S+20R) and C₂₉strerane¦Â¦Â/(¦Â¦Â+¦Á¦Á) of oil shale samples in the southern Ordos Basin vary over a small range (Tongchuan area: 0.49-0.5 and 0.36-0.43; Xunyi area: 0.45-0.46 and 0.36-0.39, respectively), demonstrating that the oil shale in these two areas has reached similar maturity. The total concentration of regular steranes is primarily composed of C₂₉(41.6%-49.0%, averaging 43.6%), with relatively low contents of C₂₇(25.0%-36.1%, averaging 30.3%) and C₂₈(19.8%-33.0%, averaging 26.1%) in all oil shale samples. The distribution and content of sterane in oil shale samples in the Tongchuan area and Xunyi area are very similar (Figure 6), indicating that the oil shale in the southern Ordos Basin has a homologous organic matter source.

Table 4 Rare earth elements contents and associated geochemical parameters of oil shale samples from Chang 7 sub-unit (Unit of rare earth elements in ug/g)

Figure 5 NASC-normalized REEs distribution pattern of oil shale samples (a) and chondrite-normalized REEs distribution pattern of oil shale samples (b). These two distribution patterns show that oil shale in these two areas formed in an appropriate environment with similar provenance

5 Discussion

5.1 Sources and maturity level of organic matter

Based on several analysis methods (rock pyrolysis, element C, H and O analysis, carbon isotope and saturated hydrocarbon mass chromatogram), the organic matter source of the oil shale from the Chang 7 sub-unit in the southern Ordos Basin is considered to be primarily algae [66-70]. However, the oil shale samples were primarily collected in outcrops, and weathering and biodegradation can have a substantial influence on the above analysis methods. In particular, when oil shale samples reach maturity, a saturated hydrocarbon mass chromatogram can wrongly reflect the source of organic matter. In a mass chromatogram of steranes, C₂₇, C₂₈ and C₂₉ regular steranes are negligibly affected by weathering and biodegradation, and can provide a good explanation of the source of organic matter [29, 30, 63]. Generally, C₂₇ and C₂₈ regular steranes are primarily from lower aquatic organisms (e.g., various algae), and the C₂₉ regular sterane is typically from advanced plants [29]. By analyzing the relative content of regular steranes, the source of organic matter in oil shale was determined to be of mixed type, indicating that the source of organic matter in oil shale from the Chang 7 sub-unit in the southern Ordos Basin is a blend of lower aquatic organisms and advanced plants (Figure 7(a)), rather than merely algae.

Figure 6 m/z 217 mass fragmentograms of saturated hydrocarbon fractions of six representative oil shale samples in Tongchuan and Xunyi areas. The left and right three oil shale samples were collected from Xunyi area and Tongchuan area, respectively, demonstrating the organic matter sources of oil shale

In addition, the maturity of the Chang 7 source rock in the southern Ordos Basin remains controversial. Based on the analysis of vitrinite reflectance (Ro), different scholars hold different opinions concerning maturity. The differences related to the use of different laboratory instruments, as well as unsystematic samples. Biomarker ratios are less affected by experimental error and can provide an accurate indication for maturity. The regular steroids 14 and 17(H) are converted from the ¦Á¦Á configuration to the ¦Â¦Â configuration during the evolution of steroid series [30, 71-72]. ¦Á¦Á¦ÁC₂₉sterane20S/(20S+20R) and C₂₉strerane¦Â¦Â/ (¦Â¦Â+¦Á¦Á) are usually used to investigate maturity, and increase with increasing degrees of maturity [3, 73]. As discussed in section 4.2, ¦Á¦Á¦ÁC₂₉sterane20S/ (20S+20R) and C₂₉strerane¦Â¦Â/(¦Â¦Â+¦Á¦Á) ratios of the oil shale samples show that the oil shale in the southern Ordos Basin has reached maturity (Figure 7(b)).

5.2 Paleoenvironment comparison

Although the Tongchuan and Xunyi areas are all in semi-deep to deep lake facies, the paleo- environment in these two areas must have been different due to the tectonic setting, leading to a distinct redox condition, paleoclimate, sedimentation rate, etc [37]. Several single elements can provide good representation of the environment, including U, Ni and V [74-79]. However, due to limitations in the application and contingency of sampling, these elements cannot accurately describe the environment variation. Element ratios are sensitive to the paleo-environment, and achieve a good effect in a lacustrine environment [75, 78]. Therefore, in the current study, several element ratios are considered in the discussion of the different environments of the two areas.

Figure 7 Ternary diagram of C₂₇, C₂₈ and C₂₉ terane compositions of oil shale samples, showing that source of organic matter of oil shale samples in southern Ordos Basin is mixed matter, comprising lower aquatic organisms and advanced plants. The base map is from YANDOKA et al [31] (a) and cross plot (b) of ¦Á¦Á¦ÁC₂₉sterane20S/(20S+20R) versus C₂₉strerane¦Â¦Â/(¦Â¦Â+¦Á¦Á) of oil shale samples, reflecting that oil shale in southern Ordos Basin has reached maturity. The base map is from LI et al [30] and DUAN et al [64] (b)

U, Ni and V are sensitive to the redox condition. Thus, U/Th is generally applied to reconstructing the redox condition; gradual increases in the ratio are associated with the redox condition turning anoxic [3, 39, 78]. Similarly, V/Ni can also reflect the redox condition; a ratio over 1 indicates an anoxic condition, whereas a ratio below 1 reflects an oxic condition [15, 78]. The V/Ni ratios of the oil shale samples were all over 1, indicating the presence of anoxic conditions during oil shale sedimentation in the southern Ordos Basin. Further, the U/Th and V/Ni ratios of oil shale samples in the Tongchuan area (averaging 3.92; 8.76) were clearly larger than those in the Xunyi area (averaging 0.73; 4.23), indicating that the degree of anoxic condition in the Tongchuan area is higher and more beneficial for organic matter preservation (Figures 8(a) and (b)).

The geochemical behavior of Sr and Ba is very similar [18, 39, 56]. However, when the mineralization of water increases together with increase in salinity, Ba deposits in the form of BaSO₄ first. Sr deposits when salinity concentrates to a certain extent [18, 39, 56]. Thus, Sr/Ba can be used to reflect salinity. Generally, Sr/Ba >1, 0.6-1, and <0.6 reflect sea water, brackish water, and fresh water, respectively [3, 78]. The Sr/Ba of oil shale samples in the study area ranged from 0.23 to 0.69, except for one sample that was over 0.6 (ZK1501H13), suggesting that the oil shale primarily deposited in fresh water. The Sr/Ba ratios of oil shale samples in the Tongchuan area (0.29-0.69, averaging 0.45) were higher than those in the Xunyi area (0.23-0.39, averaging 0.31), indicating that the paleosalinity in the Tongchuan area is slightly higher than that of the Xunyi area (Figure 8(c)). Previous studies have shown that relatively high paleosalinity is good for the preservation of organic matter [18, 76, 79]. Thus, in view of the preservation condition, the Tongchuan area is better than the Xunyi area.

Several pieces evidence, such as micro-and nannofossils [80], tuff (Figure 9(a)) [38, 39], high gamma sandstone [81], syndepositional structure (Figure 9(b)), have indicated the existence of hydrothermal fluid activities during oil shale sedimentation in the southern Ordos Basin. However, there is a lack of evidence evaluating the intensity of the hydrothermal fluid activities. Element ratios, particularly Fe/Ti, can provide evidence of hydrothermal fluid activities, as well as intensity. When Fe/Ti is over 20, it is indicated that the sediments are affected by obvious hydrothermal fluid activities, and the intensity becomes stronger with the increase in the Fe/Ti ratio [82]. The Fe/Ti ratios of the oil shale samples in the Xunyi area are all below 20 (8.68-18.37, averaging 12.96), and the ratios in the Tongchuan area are mostly over 20(15.22-37.57, averaging 28.45), with one exception (ZK1501H13). These findings provide evidence of obvious hydrothermal fluid activities in the Tongchuan area (Figure 8(d)). The hydrothermal fluid activities can bring about some saline material, which could be the main reason for the paleosalinity discrepancy between the two areas. In lacustrine sediments, hydrothermal fluid can have a positive effect, not only on the organic matter enrichment, but also on its preservation [11, 83]. Thus, oil shale in the Tongchuan area has higher organic matter abundance and more superior preservation conditions than the Xunyi area.

The Sr/Cu ratio of sediments is sensitive to climate and can be used to reconstruct the paleoclimate [18, 56]. Ratios of 1-10 and >10 indicate a warm and humid climate, and a dry and hot climate, respectively. The average value of the Xunyi and Tongchuan areas was 0.31 and 0.45, respectively, with all samples within a narrow range. This indicates that the paleoclimate in these two areas is warm and humid (Figure 8(e)).

La and Yb are differentiated by electrovalence and adsorbing ability as the environment changes [24,84]. When the detention time is short, sediments deposit sharply and the (La/Yb)_NASC approaches to 1. On the other hand, long detention time leads to adequate physicochemical reactions and the (La/Yb)_NASC deviates from 1. Figure 8(f) shows that the (La/Yb)_NASC values of samples from the Xunyi area are fundamentally higher than those from the Tongchuan area, suggesting that the sedimentary rate of the Tongchuan area is lower than that of the Xunyi area and the paleo-water-depth in the Tongchuan area is deeper than that in the Xunyi area. The low sedimentary rate is beneficial for the preservation of organic matter.

Fe and Co are congener elements with similar physicochemical properties. In a nearshore lacustrine environment, Fe deposits more than Co, and the Co concentration increases with increasing water depth [85]. Therefore, the Fe/Co ratio can indicate the relative water depth, and increases with decreasing values of the ratio. The Fe/Co ratios of the oil shale samples in the Tongchuan area (2645.76-11421.05, averaging 5868.92) are mostly lower than those in the Xunyi area (6426.23- 17919.29, averaging 9283.30), indicating that the paleo-water-depth in the Tongchuan area is deeper than that in the Xunyi area (Figure 8(g)). In addition, Th/U can also be applied to judging water depth; higher ratios indicate deeper water [85, 86].

Figure 8 Comparison of elements geochemistry parameters and paleoenvironment of oil shale samples in Tongchuan and Xunyi areas. The comparison of redox proxies (U/Th and V/Ni) indicates that redox condition during the Chang 7 oil shale sedimentation in Tongchuan area is more reducing than that of Xunyi area. The paleosalinity proxy (Sr/Ba) comparisons show that salinity of Tongchuan area is higher than that of Xunyi area, benefiting oil shale preservation. The hydrothermal fluid activities proxy (Fe/Ti) comparison illustrated that the Tongchuan area experienced stronger hydrothermal fluid effects than the Xunyi area. The paleoclimate proxy (Sr/Cu) comparison between the two areas showed no discrepancy. The sedimentary rate proxy ((La/Yb)_NASC) comparison indicated that the oil shale in the Tongchuan area deposit is more stable than that of the Xunyi area. Comparison of the paleo-water-depth proxies (Fe/Co and Th/U) demonstrated that the depth of the water in the Tongchuan area is deeper than that of the Xunyi area

Figure 9 Tuff (a) and syndepositional structure (b) of oil shale section in Yishicun profile, Tongchuan area, indicating obvious hydrothermal fluid activities in southern Ordos Basin

Oil shale samples from the Tongchuan area (0.16-0.98, averaging 0.36) show lower Th/U ratios than those in the Xunyi area (0.56-4.76, averaging 2.27) (Figure 8(h)), similar to the findings for the Fe/Co ratio. The deeper the water is, the more space there is for organic matter to deposit in. Therefore, the Tongchuan area is better for the formation and preservation of oil shale.

5.3 Depositional model

Previous studies have indicated that the paleo-productivity of the lake in the southern Ordos Basin is relatively high, with an average (TOC) in the oil shale of 16.04 wt% [14]. According to oil yield statistics, the average oil yield of the oil shale from the Tongchuan area (>5.5 wt%) is significantly higher than that from the Xunyi area (4.5 wt%-5.5 wt%) [15, 18, 56]. The oil yield of oil shale is positively correlated with TOC; the relationship can be expressed as T=2.79¡Áw+0.907, where T and w represent the TOC (wt%) and oil yield (wt%), respectively [18]. After calculating the TOC, the results indicate that the average TOC of the oil shale from the Tongchuan area is above 16.25 wt%, whereas the corresponding value from the Xunyi area is below this; this may be a result of the sedimentary environment differences between the areas. During oil shale sedimentation, tectonic and volcanic orogenies in the southern Ordos Basin play an important role in the formation of oil shale. The liquefied sandstone vein that can be seen in sections of the oil shale further supports this (Figure 10(a)). With regard to plate movement, the Triassic Indosinian movement causes the Qinling paleooceanic to subduct to the North China Plate, forming an active continental margin with the trench-arc-basin system [87]. The southern Ordos Basin then gradually subsides. During the Chang 7 period, the depression reaches its maximum, receiving a large amount of organic matter and terrestrial detrital material, which primarily comes from the south. The siltstone granularity analysis of the oil shale interlayer indicates that part of the clastic rocks entering into the lake are in the form of turbidite [88], and higher terrestrial plant fossils can be seen in the sandstone interlayers (Figure 10(b)). This suggests that higher terrestrial plants are accompanied by tectonic orogenies, entering into the lake. Thus, organic matter input of the oil shale is dominated by algae and terrestrial plants, contributing to the material base of the oil shale. The hydrothermal fluid activity caused by volcanic eruption plays an important role in the enrichment of the organic matter in the oil shale. HE et al [83] and HE et al [11] suggested that hydrothermal fluid activities can not only supply nutrient elements, but can also enhance the reducibility of the lake, which is beneficial to oil shale preservation. Further, thermal fluid accelerates the death of organisms, which also has a positive effect on the enrichment of organic matter in oil shale [65-70]. The strong reducing condition of the water body is also conducive to the preservation of the organic matter. The relatively deep water provides not only low oxygen, but also weak hydrodynamic force, and the stable sedimentary condition is good for biodegradation. The warm and humid climate is beneficial to the biological development, providing a favorable physical basis for the oil shale. Influenced by the reducing condition, thermal fluid activities, slow deposition rate, warm and humid paleoclimate, and fresh water, the oil shale from the Chang 7 sub-unit in the southern Ordos Basin is well developed (Figure 11). In view of the paleoenvironment, the oil shale in the Tongchuan area is superior to that in the Xunyi area, suggesting that the Tongchuan area is the key exploration area in the southern Ordos Basin.

Figure 10 Liquefied sandstone vein (a) and higher plant fossils (b) across the oil shale section in Yishicun profile, Tongchuan area. Liquefied sandstone vein results from an earthquake event, suggesting intense tectonic and volcanic orogenies. Higher plant fossils occur with turbidite, implying that event deposits are the main pattern for higher plants entering the lake

Figure 11 Depositional model of Chang 7 oil shale of Triassic Yanchang Formation in Tongchuan and Xunyi areas in southern Ordos Basin. The oil shale deposited in warm and humid weather with a relatively light water body. From the Tongchuan area to Xunyi area, hydrothermal fluid activities, and reducing condition and terrigenous input gradually decrease with increasing deposition rate. The organic matter sources of oil shale are dominated by algae and terrestrial plants, with a superior preservation environment facilitating the formation of the oil shale

Table 5 Biomarker parameters of m/z 217 of oil shale samples

Table 6 Elements geochemistry parameters of paleoenvironment of oil shale samples in Tongchuan and Xunyi areas

6 Conclusions

This paper compares the depositional conditions of oil shale from the Tongchuan and Xunyi areas of the Chang 7 sub-unit, and summarizes a depositional model of the oil shale from the southern Ordos Basin. Several conclusions are outlined below:

1) Element geochemistry characteristics indicate that the Al₂O₃/TiO₂ ratios of oil shale samples range between 21.67 and 46.69, illustrating that the parent rock of oil shale in the southern Ordos Basin is felsic igneous rocks. Si/(Si+Al+Fe) ratios of the oil shale samples suggest that the sediment location of the oil shale is not far from the terrigenous provenance. U and Cu are enriched in the oil shale samples, indicating that the content of organic matter in the oil shale is high. The concentration and distribution pattern of rare earth elements indicates that the oil shale in the southern Ordos Basin is deposited in a similar paleoenvironment. However, the characteristics of the major elements, trace elements, and rare earth elements in oil shale samples from the Tongchuan and Xunyi areas are significantly different, which may reflect the depositional conditions.

2) The sterane distribution of six oil shale samples is very similar, and the C₂₉ concentration is higher than the C₂₇ and C₂₈ concentrations, suggesting that the oil shale in the southern Ordos Basin has a homologous organic matter source, with the organic matter being of mixed origin comprising lower aquatic organisms and advanced plants. ¦Á¦Á¦ÁC₂₉sterane20S/(20S+20R) and C₂₉strerane¦Â¦Â/(¦Â¦Â+¦Á¦Á) ratios of the oil shale samples vary only slightly (Tongchuan area: 0.49-0.5 and 0.36-0.43; Xunyi area: 0.45-0.46 and 0.36-0.39, respectively), and indicate that the oil shale has reached maturity.

3) U/Th and V/Ni ratios suggest that the redox condition is dominated by a reducing condition, and the anoxic degree of the Tongchuan area is higher than that of the Xunyi area. Sr/Ba ratios illustrate that the oil shale is deposited in fresh water and the paleosalinity in the Tongchuan area is slightly higher than that in the Xunyi area. Fe/Ti ratios imply that the Tongchuan area has gone through obvious hydrothermal fluid activities, while the Xunyi area has not. Sr/Cu ratios indicate a warm and humid paleoclimate in both areas, with no obvious difference between the two areas. The (La/Yb)_NASC indicates that the deposition rate in the Tongchuan area is lower than that in the Xunyi area. Fe/Co and Th/U ratios indicate that the paleo-water-depth in the Tongchuan area is deeper than that in the Xunyi area.

4) In light of the paleoenvironment of oil shale in the southern Ordos Basin, a depositional model of the oil shale is proposed. Compared with the Xunyi area, the Tongchuan area appears to be the key exploration area for the oil shale in the Chang 7 sub-unit of the southern Ordos Basin.

Appendix A:

Peak assignments for m/z 217 mass fragmentograms of saturated hydrocarbon fractions

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(Edited by FANG Jing-hua)

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Foundation item: Projects(41173055, 41772118) supported by the National Natural Science Foundation of China

Received date: 2018-12-18; Accepted date: 2019-02-27

Corresponding author: LI De-lu, PhD, Lecturer; Tel: +86-18402990271; E-mail: lidelu322@126.com; ORCID: 0000-0003-2211-6840